There are numerous composite rivets available but most are presently designed for use with thermosetting resin matrix materials. Basically, a thermosetting resin is one that is heated to a specific temperature, usually above the melting point, wherein it undergoes a chemical change (cures) and strengthens. Since thermosetting resins will polymerize even at room temperatures, they must be stored at low temperatures. On the other hand, the strength of thermoplastic resins are inherent and they do not undergo a chemical change when heated. This characteristic coupled with the fact that many thermoplastics can function at temperatures above 500.degree. F. make them ideal matrix material for composites.
Composite rivets utilizing thermosetting resin matrixes are installed and "upset" while in a soft, partially cured "B" stage condition, so that they can be deformed. The amount and duration of heat to be applied is a function of the particular "B" stage resin utilized. However, even in this "B" stage they must be kept at low temperature to prevent further curing over a period of time. For example, U.S. Pat. No. 4,478,543 "Blind Rivet" by Lyon discloses a rivet having a fully cured head and shear portion (shank) and a separate partially cured (B stage) head forming portion held together by a centrally disposed mandrel. The head forming portion is deformed after heating by tensioning the mandrel. Thereafter, the temperature of the head forming portion is raised to the curing temperature. Another example can be found in U.S. Pat. No. 4,478,544 "Composite Rivet" by Strand. This is a composite rivet wherein carbon fibers are encapsulated in a "B" stage resin matrix and are enveloped in a high tensile strength and low modulus tubular sheath. The sheath restricts the flow of the thermosetting resin during the cure cycle. The problem with the two rivets described above is that the use of a thermosetting resin matrix material provides only limited shelf life. A metal mandrel can cause corrosion problems especially if the filamentary material is graphite. Additionally, no effort is made to prevent breakage or damage of the filamentary material when the head forming portion of the rivet is upset.
U S. Pat. No. 4,687,396 "One-Piece Composite Rivet with Deformable Head Portion and Mandrel" by Berecz discloses a one-piece rivet wherein the head forming portion incorporates an integral mandrel extending therefrom. The head is formed by shearing away material from the mandrel and compressing it to form an "upset" head. U.S. Pat. No. 4,687,397 "Composite Rivet with Strippable Mandrel" by Berecz is similar except that a retaining ring is provided to limit the amount of deformation of the sheared off portion of the mandrel. In these two examples, the deteriorating effect of the shearing action on the filamentary material is questioned.
U.S. Pat. No. 4,687,398 "Composite Rivet with Collar Reinforced with Circumferential Fibers" by Berecz also discloses a rivet which uses an axially compressible radially expandable tubular preform with a mandrel extending therethrough. The mandrel incorporates an annular recess which extends from the end of the structures being joined and a locking collar is mounted thereabout. Upon compression of the head forming portion and tensioning of the mandrel, material is forced between the annular groove of the mandrel and the locking collar, with the locking collar limiting deformation. In U.S. Pat. No. 4,687,395 "Composite Rivet with Deformable Annular Collar Containing Randomly Chopped Fibers" by Berecz a similar concept is disclosed except the mandrel is integral and extends from the end of the preform. In this design, a locking collar is swaged into a recess that exists in the head forming portion of the mandrel. No locking collar is used. In these latter examples, no provision is made for removing the excess mandrel material. In addition, the rivet preforms have complex shapes and again no provision for preventing breakage of the filamentary material in the head forming portion is provided. Furthermore, these rivets require special installation tools.
Thus, it is a primary object of the subject invention to provide a composite rivet that can use a thermoplastic resin as the matrix material.
It is another primary object of the subject invention to provide a composite rivet which can be installed using only slightly modified existing installation tools.
It is further object of the subject invention to provide a composite rivet that provides controlled deformation of the filamentary material in the head forming portion thereof so that damage to the filamentary material is minimized.
It is a still further object of the subject invention to provide a composite rivet wherein the selected materials used in the manufacture thereof minimize the possibility of corrosion.